Coaching and scheme. The talent gap should have been the widest when we played OU close from 2012-2015, when our upperclassmen were recruited during the MWC days. Then Riley gets there and OU puts up 52, 38, 41, and 52 points, and, outside of a comeback in 2016 after trailing 49-24, the games haven't been close. He's found something in GP's defense that GP hasn't been able to adjust to yet.
The fact that their OL holds on literally every play, knowing the refs will come to see it as "normal" and thus won't call it, is definitely scheme as well.